Typically a thin film evaporator comprises a vertical or horizontal drum, a supply line which is used to supply the product that is to be evaporated, a heating jacket arranged on the periphery of the drum, a discharge line for discharging the residue left and a discharge line for discharging the evaporated portion of the product. The overall purpose of the thin film evaporator is to evaporate volatile fluids from less volatile fluids. The evaporation occurs by the contact between the product and the heated walls of the drum. To improve the evaporation efficiency, the drum is provided with an agitation means. The agitation may be made in a number of ways. One well known type is an agitator of the fixed clearance type in which a thin gap is formed between the agitator and the inner wall of the drum, whereby the agitator during rotation forces the product towards the thin gap. Another type is a so called wiped film agitator, in which the agitator wipes the product against the inner wall of the drum, thereby forming a thin product film.
Conventionally, the only way to add heating surface is either to make the drum diameter larger or to make the drum longer. This adds to the bulkiness and cost of the evaporator.
It is also possible to increase the capacity and reduce energy consumption by adding a separate, stand alone pre-heater, e.g. a heat exchanger, to the process. Thereby the product is preheated before it enters the drum. The stand alone pre-heater is arranged in a position upstream the evaporator.
Typically, the inside heated wall of a thin film evaporator is designed a lot thicker than most heat exchangers due to the relatively large inside diameter as compared to a standard shell and tube heat exchanger. To compensate for the larger wall thickness and reduced heat transfer caused thereby while still allowing a speedy evaporation with short residence time as the product meets the heated inner wall of the drum, the heating medium used in the heating jacket often has a large temperature difference when entering the drum. However, this causes the drawback of losing energy to the outside environment via the wall portion of the heating jacket which faces away from the drum unless that wall is thoroughly insulated. In fact, even with a thorough insulation undue and costly energy losses will occur.
These problems are equally applicable to a so called short path evaporator. A short path evaporator basically consists of a cylindrical drum with a heating jacket, an agitator and a condenser inside. The agitator creates by rotation a thin product film on the heating surface inside of the drum. By means of gravity the product flows in a spiral path downwards, whereby the volatile portion of the product evaporates. The vapors passes the shortest route and with practically no pressure drop to the internal condenser where it is condensed. The non-volatile portion reaches the lower portion of the drum and leaves it via a product outlet. Like in prior art agitated thin film evaporators or wiped film evaporators, it is well established technology to use a stand-alone pre-heater which is arranged in a position upstream of the evaporator to thereby feed the evaporator with a preheated product.
Thus, there is a need for a more energy efficient solution with reduced energy losses. Also there is a need to reduce the overall cost of an installation by allowing reducing or even eliminating the need for an external pre-heater. The technology should be equally applicable no matter if it is applied to an agitated thin film evaporator, a wiped film evaporator, a short path evaporator or a thin film dryer.